The Singapore-based campaign by conservation group WWF was boosted by Hollywood star power, with "The Amazing Spiderman-2" stars Andrew Garfield, Emma Stone and Jamie Foxx leading ceremonies at the city-state's Marina Bay district.

Comic-book hero Spiderman is this year's "ambassador" for Earth Hour, which was launched in Sydney in 2007.

Sydney's Opera House and Harbour Bridge were among the first landmarks around the world to dim their lights for 60 minutes during Saturday's event.

An estimated 7,000 cities and towns from New Zealand to New York are taking part.

Hong Kong's stunning waterfront skyline was unrecognisable on Saturday evening, with the city's tallest skyscraper, the International Commerce Centre, stripped of the vast light show usually wrapped around its 118 stories.

Blazing neon signs advertising some of the world's largest brands were shut off, leaving the view of the heavily vertical southern Chinese city peppered only with tiny lights from buildings' interiors.

Earth Hour partnered with payments giant PayPal to allow donors to contribute to specific projects from Russia and India to Canada and Indonesia, using Asian fundraising site Crowdonomic.

Earth Hour chief executive Andy Ridley said before the lights went off in Singapore that the event had moved beyond symbolism to concrete action.

"If you want to get real social change you need to have symbolism," he told AFP.

"We are seeing some really big outcomes."

Projects under the "Earth Hour Blue" crowdfunding scheme -- which aim to raise more than $650,000 in total -- include a turtle centre in Italy and funding for forest rangers in Indonesia.

The event is being marked in more than 150 countries, organisers said, estimating that thousands of cities and towns would have taken part by the time the ceremonies began in Singapore.

The projects seeking crowdfunding include a 24,000-dollar effort in the Philippines to bring fibreglass boat technology to coastal communities affected by super typhoon Haiyan in November last year.

In Nepal, $100,000 is being sought for a programme called "A Flame Called Hope" to provide access to biogas energy for 150 households in the Terai region, reducing the need for wood as fuel and helping protect the habitat of endangered wildlife, according to the Earth Hour website.

Spiderman-2 star Garfield told journalists that he was a personal supporter of the Nepal project.

"What they are doing is turning waste into energy, it's like the cycle of life right there, if only everyone knew how simple it was," he said.

Earth Hour will see other landmarks including the Empire State Building in New York, the Eiffel Tower in Paris and the Kremlin in Moscow switch off their lights for an hour starting at 8:30 pm local time on Saturday.